You should understand the GMAT structure and standards. From mba.com you can download 2 practice test. These 2 practice tests contain genuine GMAT questions, and are best at assessing your GMAT score.
Take a practice test to figure out how far you are from your objective. The main reason for taking test is that you can see, you’re beginning stage (now) and afterward analyze it against the completion (taking GMAT).
Identify your strength and weakness. Figure out the areas where you are making mistakes, why you are making mistakes which and which part is more difficult for you.
Prepare a study plan after knowing your strength and weakness. Get books for preparation, if possible do a course.
Don’t take the test without preparation, hoping for some miracle to happen.